The invention relates to micro-machined silicon accelerometers and in particular, to mechanical packaging and electrical signal routing.
The oil and gas well drilling and survey fields as well as many other fields of industry require low cost, highly accurate and highly reliable measuring instruments and sensors. Measuring instruments and sensors used in the oil and gas well drilling and survey fields and many other industries, for example, aircraft equipment and military ordinance, must survive and operate accurately in extreme environmental conditions including, for example, extreme vibrations and impacts experienced at temperatures ranging from well below freezing to hundreds of degrees Centigrade. In these applications, measuring instruments and sensors are often tested for operation in random vibration environments as extreme as 20 g RMS to 1,000 Hz and sine vibration environments as extreme as 30 g peak; shock or impact environments in the range of 1,000 g's while surviving shocks of as much as 2,000 g's; and temperature extremes ranging from -40 to +200 degrees Centigrade.
In the oil and gas industry, sensor packaging and environmental needs have become even more severe as deposits are sought in ever deeper boreholes. The deeper the borehole the more extreme the temperature in which the drill string and drill string steering tools must operate. Boreholes frequently also contain changes in direction at various points along its depth. The deep and angular boreholes often require the measuring instrument or sensor guiding the drill string to operate in very small spaces which may be one inch or less in diameter. Current directional instruments are too large and temperature limited to operate effectively in these applications. For example, current instruments are typically 11/4 to 11/2 inch in diameter and are typically limited to operate in temperature environments of +150 degrees Centigrade or less.
Well drilling and logging operations typically require that very expensive equipment and highly skilled workers operate in remote locations. This combination of factors results in operating costs that may run half a million dollars or more per day. Thus, an equipment failure which forces operations to shut-down may be very costly. To limit the costs associated with equipment failures, many operators keep spare parts on hand, including spare measuring instruments and sensors, even though the spare parts may be very expensive themselves. Such costly operations in such extreme environmental conditions demand sensors and measuring instruments which are compact, very rugged and highly reliable.
In order to survive and operate in such hostile environments while meeting the needs imposed by the various drilling, survey and industrial applications, manufacturers of high accuracy measuring instruments and sensors have employed various packaging methods to protect the sensor. High accuracy sensor packaging methods include packaging the sensor element within a housing and sealing the housing with a header having interconnect pins for inputting power and drive signals and outputting sensor signals. The housing consists of a metal container, for example, a deep drawn metal can or machined metal housing. The header, also manufactured of metal, uses metal interconnect pins to provide power and signal routing interfaces. The interconnect pins are typically sealed in the header using conventional glass sealing technology.
Besides the extreme vibration and shock environments, the external temperature in which the instrument must operate may adversely impact the operation of the internal sensor. For example, the difference in temperature expansion between the silicon sensor element and the metal housing/header arrangement introduces strain at the mounting interface and may result in measurement errors. Many exotic methods have been used in attempts to prevent external environmental factors from disturbing or interfering with the operation of the internal sensor. However, use of instruments in these environmentally hostile environments nonetheless remains temperature limited.